UNIVERSITY   OF   CALIFORNIA 

AGRICULTURAL  EXPERIMENT  STATION 
E.   W.    HILGARD,    DIRECTOR 


THE  CONSERVATION  OF  SOIL  MOISTURE 


AND 


ECONOMY  IN  THE  USE  OF  IRRIGATION  WATER 


BULLETIN   121 


BERKELEY 

£be  TUniversitE  press 

AUGUST,   1898 


THE  CONSERVATION  OF  SOIL  MOISTURE 

AND 

ECONOMY  IN  THE  USE  OF  IRRIGATION  WATER. 


By  E.  W.  Hilgard  and  R.  H.  Loughridge. 


The  exceptionally  dry  season  of  1897-8,  coupled  with  the  early 
cessation  of  rains  in  the  spring  of  1897,  have  brought  about  in  Cali- 
fornia a  more  extended  failure  of  cereals  and  pasturage,  and  shallow- 
rooted  crops  generally,  than  in  any  year  since  the  State  became  a 
prominently  agricultural  one  the  season  of  1876-7  being  the  nearest 
to  carry  with  it  a  similar  deficiency  in  crop  production.  It  has  been 
the  effort  of  the  Experiment  Station  to  utilize  the  present  unusual 
season  for  the  study  of  the  limits  of  endurance  of  drought  on  the  part 
of  the  several  crop  plants,  and  with  it  to  determine  the  minimum  of 
water  that  will  suffice  for  their  satisfactory  growth  in  the  several  soils. 
While  far  from  completed,  this  work  (involving  many  hundreds  of 
determinations  of  moisture  in  soils)  has  already  yielded  some  results 
which  render  it  desirable  that  they  should  be  placed  before  the  farmers 
and  discussed  at  once,  in  order  to  provide  against  a  recurrence  of 
avoidable  injury  in  the  future. 

Amount  of  water  required  by  crops. — It  is  not  very  generally 
understood  how  large  an  amount  of  water  is  required  for  the 
production  even  of  fair  crops;  for  the  maximum  of  possible 
product  is  rarely  obtained  on  the  large  scale,  because  it  is  not 
often  that  all  conditions  are  at  their  best  at  anyone  time  and  locality. 
But  from  numerous  observations,  made  both  in  Europe  and  in  the 
Eastern  United  States,  it  has  been  found  that  from  300  to  over  500 
tons  of  water  are  on  the  average  required  to  produce  one  ton  of  dry 
vegetable  matter.  In  Wisconsin,  King  found  that  a  two- ton  crop  of 
oat  hay  required  over  one  thousand  tons  of  water  per  acre,  equal  to 
about  nine  inches  of  rainfall.  The  average  rate  for  field  crops  at 
large  is  given  by  European  observers  at  325  times  the  weight  of  dry 
matter  produced,  being  at  the  rate  of  about  three  inches  of  rainfall 
actually  evaporated  through  the  plant. 

These  data  should  enable  us  to  estimate  the  adequacy  of  the 
moisture  contained  in  the  soil  at  the  beginning  of  the  dry  season  to 
mature  the  crop,  provided  we  make  due  allowance  for  any  growth 
already  made  at  the  time,  and  provided  also  that  the  estimates  as  to 
the  water-requirements  derived  from  the  experience  of  the  countries 
of  summer  rains  (the  humid  regions)  hold  good  for  the  arid  region 
also.     Whether  or  not  this  can  be  assumed,  is  among  the  points  our 


experiments  are  designed  to  determine.  The  surprisingly  successful 
growth  and  bearing  especially  of  deciduous  trees,  without  irrigation, 
despite  a  drought  of  five  or  six  months  in  the  "Franciscan  climate,"* 
has  led  to  an  impression  that  a  less  amount  of  water  may  suffice 
under  arid  conditions.  For  in  the  East,  as  many  weeks  of  drought 
and  intense  heat  would  frequently  suffice  to  destroy  the  crop. 

Probable  causes  of  this  endurance  of  drought. — Doubtless  the 
main  cause  of  this  remarkable  endurance  is  to  be  found  in  the  much 
deeper  rooting  of  all  plants  in  arid  climates;  whereby  not  only  a 
much  larger  bulk  of  moist  soil  is  at  their  command,  but  the  roots  are 
withdrawn  from  the  injurious  effects  of  the  hot,  dry  surface  and  air. 

This  deeper  range  of  the  roots  is  not  the  result  of  foresight  on 
the  part  of  the  plant.  It  could  not  occur  on  Eastern  soils,  because  of 
the  intervention,  in  the  great  majority  of  cases,  of  difficultly  pene- 
trable subsoils;  from  which,  moreover,  plants  could  draw  but  little 
nourishment  on  account  of  their  "rawness."  In  the  arid  region,  as  a 
rule,  subsoils  in  the  Eastern  sense  do  not  exist;  the  soil  mass  is 
practically  the  same  for  several  feet,  and  in  the  prevalent  soils  is  very 
readily  penetrable  to  great  depths.  This,  summarily  speaking,  is 
due  to  the  slight  formation  of  clay,  and  the  rarity  of  heavy  rains,  in 
the  arid  region.  And  this  easy  penetrability  of  the  soil  implies, 
moreover,  that  being  well  aerated,  the  depths  of  the  soil  are  not 
"raw,"  as  in  the  East;  and  therefore  that  the  "subsoil,"  such  as  it  is, 
may  fearlessly  be  turned  up  as  deeply  as  the  farmer  is  willing  to  go 
with  the  plow,  without  danger  of  injuring  the  next  season's  crop,  in 
all  lands  that  are  well  drained;  as,  by  reason  of  their  depth  and 
perviousness,  is  the  case  with  most  California  soils. 

The  accompanying  plate  illustrates  from  nature  the  deep  penetration 
of  a  peach  root  developing  in  a  normally  deep,  well  aerated  "bench" 
soil,  in  a  manner  quite  impossible  to  the  same  root  when  growing  in 
land  underlaid,  as  are  most  Eastern  ones,  by  a  subsoil  which  either  is 
too  dense  or  too  wet  to  be  penetrated  and  utilized  by  the  tree. 

A  glance  at  the  figures  suffices  to  show  that  while  a  root  system  like 
plate  1,  a  typical  Eastern  tree  root  (as  given  by  Thomas'  Fruit  Cul- 
turist,  page  82)  will  stand  in  absolute  need  of  frequent  rains  or 
irrigation  to  sustain  its  vitality,  such  a  one  as  plate  2  may  brave 
prolonged  drought  with  impunity,  being  independent  of  surface  con- 
ditions, and  able  to  perform  all  its  functions  out  of  reach  of  stress 
from  lack  of  moisture. t     It  is  equally  clear  that  it  is  to  the  farmer's 


*This  name  has  been  felicitously  applied  by  Powell  to  the  climate  of  middle  and  southern 
California,  which  is  characterized  by  the  concentration  of  rains  within  a  winter  which  is  mild  enough 
to  constitute  a  growing  season,  while  the  summer  is  practically  rainless. 

tThe  moisture  determination  under  this  tree  gave,  to  the  depth  of  eight  feet,  an  aggregate  amount 
of  water  of  1,058  tons  per  acre. 


Plate  2. 

PRUNE  TREE  ON  PEACH  ROOT.   NILES,  CAL. 


interest  to  favor,  to  the  utmost,  this  deep  penetration  of  the  roots, 
both  in  the  preparation  and  tillage  of  the  ground,  and  in  the  use  of 


— ^-^AJ^yfitfi 


Plate  1. 

ROOT    OF   AN    EASTERN    FRUIT    TREE. 

irrigation  water.  For  if  the  latter  is  used  too  frequently  or  too 
abundantly,  the  salutary  habit  of  deep  rooting  will  be  abandoned 
by  the  plant,  and  it  will,  as  in  the  East,  be  dependent  upon  frequent 
rain  or  irrigation;  and  also,  owing  to  the  small  bulk  of  soil  upon 
which  it  can  draw  for  its  nourishment,  upon  frequent  and  abundant 
fertilization. 

Eastern  immigrants  as  well  as  a  large  proportion  of  California 
farmers  do  not  realize  the  privilege  they  possess  of  having  a  triple 
and  quadruple  acreage  of  arable  soil  under  their  feet,  over  and  above 
the  area  for  which  their  deeds  call;  and  they  tenaciously  continue  to 
adhere  to  precautions  and  practices  which,  however  salutary  and 
necessary  in  the  region  of  summer  rains,  do  not  apply  to  this  climate. 
The  shallow  plowing  so  persistently  practiced  results  in  the  formation 
of  a  "  plowsole"  that  plays  the  part  of  the  Eastern  subsoil  in  preventing 
root  penetration;  limiting  their  range  for  moisture  and  plant  food, 
and  thus  naturally  causing  crops  to  succumb  to  a  slight  stress  of 
season  which  ought  to  have  passed  without  injury,  had  the  natural 
conditions  been  taken  into  proper  consideration. 

Roots  follow  moisture. — Very  striking  examples  of  deep  rooting  as 
the  result  of  vertical  moisture  penetration  can  be  observed  in  some  of 
our  native  trees,  which,  while  naturally  at  home  on  moist  ground,  are 
nevertheless  sometimes  found  forming  luxuriant  clumps  on  the  slopes 
and  even  summits  of  our  coast  ranges  and  foothills.  If  we  examine 
the  ground  where  this  occurs  in  the  case  of  California  laurel,  we  will 
generally  find  that  the  soil  in  which  they  grow  is  underlaid  by  slate  or 
shale  standing  on  edge,  into  the  crevices  of  which  the  roots  penetrate, 
wedging  them  open;  while  themselves  flattening  out,  and  thus  pene- 
trating to  moisture  at  considerable  depths.  The  same  may  be  observed 
in  the  case  of  the  erect  "bedrock"  or  foothill  slates  of  the  Sierra,  on 
which  native  as  well  as  fruit  trees  flourish  in  very  shallow  soils,  some- 
times reaching  permanent  moisture  at  the  depth  of  ten  or  more  feet 
below  the  surface.  It  can  readily  be  observed  during  rains  that  there 
is  comparatively  little  run- off  from  the  surface  of  these  lands  underlaid 
by  vertical  shales. 


8 

On  the  same  principle,  the  grape  vines  which  bear  some  of  the 
choicest  raisins  of  Malaga  on  the  arid  coastward  slopes,  are  made  to 
supply  themselves  with  moisture,  without  irrigation,  by  opening  around 
them  large,  funnel-shaped  pits,  which  remain  open  in  winter  so  as  to 
catch  the  rain,  causing  it  to  penetrate  downward  along  the  tap-root  of 
the  vine,  in  clay  shale  quite  similar  to  that  of  the  California  Coast 
Ranges,  and  like  this  latter,  almost  vertically  on  edge.  Yet  on  these 
same  slopes  scarcely  any  natural  vegetation  now  finds  a  foothold. 

Similarly  the  "ryots"  of  parts  of  India  water  their  crops  by  applying 
to  each  plant  immediately  around  the  stem  such  scanty  measure  of  the 
precious  fluid  as  they  have  taken  from  wells,  often  of  considerable 
depth,  which  form  their  only  source  of  water-supply.  Perhaps  in 
imitation  of  these,  an  industrious  farmer  has  practiced  a  similar  system 
on  the  high  benches  of  Kern  River,  and  has  successfully  grown 
excellent  fruit  for  years,  on  land  that  originally  would  grow  nothing 
but  cactus.  Sub-irrigation  from  pipes  has  been  applied  in  a  similar 
manner. 

The  principle  flowing  from  the  above  is  simply  that  the  most 
economical  mode  of  using  irrigation  water  is  to  put  it  "where  it  will  do 
the  most  good,"  close  to  the  stem  of  the  plant  or  trunk  of  the  tree,  and 
let  it  soak  downward  so  as  to  form  a  moist  path  for  the  roots  to  follow 
to  the  greatest  possible  depth.  It  is  this  deep  penetration  to  natural 
moisture,  as  a  matter  of  fact,  which  enables  the  small  quantities  sup- 
plied to  produce  such  marked  effects. 

Basin  irrigation. — It  will  be  noticed  that  this  principle  is  practically 
the  same  as  that  of  the  basin  irrigation  of  orchards,  which  was 
originally  largely  practiced  in  California,  but  has  now  been  mostly 
abandoned  for  furrow  irrigation .  The  latter  has  been  almost  univer- 
sally adopted,  partly  because  it  requires  a  great  deal  less  hand-labor, 
partly  under  the  impression  that  the  whole  of  the  soil  of  the  orchard 
is  thus  most  thoroughly  utilized;  partly  also  because  of  the  injurious 
effect  upon  trees  produced  at  times  by  basin  irrigation. 

The  explanation  of  such  injurious  effects  is,  essentially,  that  cold 
irrigation  water  depresses  too  much  the  temperature  of  the  earth 
immediately  around  the  roots,  and  thus  hinders  active  vegetation  to 
an  injurious  extent,  sometimes  so  as  to  bring  about  the  dropping  of 
the  fruit.  This,  of  course,  is  a  very  serious  objection,  to  obviate 
which  it  might  be  necessary  to  reservoir  the  water  so  as  to  allow  it  to 
warm  before  being  applied  to  the  trees.  In  furrow  irrigation  the 
amount  of  soil  soaked  with  the  water  is  so  great  that  the  latter  is  soon 
effectually  warmed  up,  besides  not  coming  in  contact  too  intimately 
with  the  main  roots  of  the  tree;  along  which  the  water  soaks  very 
readily  when  applied  to  the  trunk,  thus  affecting  their  temperature  much 


9 

more  directly.  It  is  for  the  fruit-grower  to  determine  which  consider- 
ation should  prevail  in  a  given  case.  If  the  water-supply  be  scant  and 
warm,  the  most  effectual  use  that  can  be  made  of  it  is  to  apply  it 
immediately  around  the  trunk  of  the  tree,  in  a  circular  trench  dug  for 
the  purpose.  When,  on  the  contrary,  irrigation  water  is  abundant 
and  its  temperature  low,  it  will  be  preferable  to  practice  furrow  irri- 
gation, or  possibly  even  flooding.  As  to  the  more  complete  use  of  the 
soil  under  the  latter  two  methods,  it  must  be  remembered  that  while 
this  is  the  case  in  a  horizontal  direction,  yet  unless  irrigation  is 
practiced  rather  sparingly  under  the  furrow  system,  it  may  easily 
happen  that  the  gain  made  horizontally  is  more  than  offset  by  a 
corresponding  loss  in  the  vertical  penetration  of  the  root- system. 
This  is  amply  apparent  in  some  of  the  irrigated  orange  groves  of 
Southern  California,  where  the  fine  roots  of  the  trees  fill  the  surface 
soil  as  do  the  roots  of  maize  in  a  corn  field  of  the  Mississippi  States; 
so  that  the  plow  can  hardly  be  run  without  turning  them  up  and 
under.  In  these  same  orchards  it  will  be  observed,  in  digging  down, 
that  at  a  depth  of  a  few  feet  the  soil  is  too  water-soaked  to  permit  of 
the  proper  exercise  of  the  root  functions,  and  that  the  roots  existing 
there  are  either  inactive  or  diseased.  That  in  such  cases  abundant 
irrigation  and  abundant  fertilization  alone  can  maintain  an  orchard  in 
bearing  condition,  is  a  matter  of  course;  and  there  can  be  no  question 
that  a  great  deal  of  the  constant  cry  for  the  fertilization  of  orchards 
in  the  irrigated  sections  is  due  quite  as  much  to  the  shallowness  of 
rooting  induced  by  over- irrigation,  as  to  any  really  necessary 
exhaustion  of  the  land.  When  the  roots  are  induced  to  come  to  and 
remain  at  the  surface,  within  a  surface  layer  of  eighteen  to  twenty 
inches,  it  naturally  becomes  necessary  to  feed  these  roots  abundantly, 
both  with  moisture  and  with  plant  food.  This  has  as  naturally  led  to 
an  over-estimate  of  the  requirements  of  the  trees  in  both  respects. 
Had  deep  rooting  been  encouraged  at  first,  instead  of  over-stimulating 
the  growth  by  surface  fertilization  and  frequent  irrigation,  some 
delay  in  bearing  would  have  been  amply  compensated  for  by  less  of 
current  outlay  for  fertilizers,  and  less  liability  to  injury  from 
frequently  unavoidable  delay,  or  from  inadequacy,  of  irrigation. 

Conservation  of  Soil  Moisture. — Alongside  of  economy  in  the 
use  of  irrigation  water,  the  conservation  of  the  moisture  imparted  to 
the  soil  either  by  rains  or  irrigation  is  most  important;  Critically  so 
where  irrigation  is  unavailable. 

Utilization  of  winter  rains,  and  winter  irrigation. — However  strong 
is  the  popular  demand  for  storage  of  the  winter  rainfall  and  flood 
waters,  too  many  do  not  appreciate  the  importance  of  the  storage  they 
can  command  without  the  use   of  reservoirs,   within  their  own  soil 


10 

mass.  While  there  is  a  well-grounded  objection  to  subjecting  plowed 
land  to  the  leaching  action  of  the  abundant  rains  in  the  humid  region, 
no  such  objection  holds  in  the  case  of  lands  lying  within  the  limits  of 
20  to  25  inches  of  annual  rainfall.  Here  the  absorption  of  the  winter 
rains  should  be  favored  to  the  utmost,  for  the  run-off  is  mostly  a  dead 
loss.  Fall  plowing  wherever  the  land  is  not  naturally  adaquately 
absorbent,  and  is  not  thereby  rendered  liable  to  washing  away,  is  a 
very  effectual  mode  of  utilizing  the  winter's  moisture  to  the  utmost, 
so  as  to  bring  about  the  junction  of  the  season's  moisture  with  that  of 
the  previous  season,  which  is  generally  considered  as  being  a  condition 
precedent  for  crop  production  in  dry  years.  The  same  of  course  holds 
true  of  winter  irrigation;  the  frequent  omission  of  which  in  presence 
of  a  plentiful  water  supply  at  that  season  is  a  prolific  cause  of  avoidable 
crop  failures.  Moistening  the  ground  to  a  considerable  depth  by 
winter  irrigation  is  a  very  effective  mode  of  promoting  deep  rooting, 
and  will  thus  stand  in  lieu  of  later  irrigations,  which,  being  more 
scant,  tend  to  keep  the  roots  near  the  surface. 

Knowledge  of  the  subsoil. — It  cannot  be  too  strongly  insisted  upon 
that  in  our  arid  climate  farmers  should  make  themselves  most 
thoroughly  acquainted  with  their  subsoil  down  the  depth  of  at  least  four, 
but  preferably  six  or  eight  feet.  This  knowledge,  important  enough 
in  the  East,  is  doubly  so  here,  since  all  root  functions  are  and  must  be 
carried  on  at  much  greater  depths.  It  is  hardly  excusable  that  a 
business  man  calling  himself  a  farmer  should  omit  the  most  elementary 
precaution  of  examining  his  subsoil  before  planting  orchard  or  vine- 
yard, and  should  at  the  end  of  five  years  find  his  trees  a  dead  loss  in 
consequence  of  an  unsuitable  subsoil.  Similarly,  no  irrigator  should 
be  ignorant  of  the  time  or  amount  of  water  it  takes  to  wet  his  soil  to 
a  certain  depth.  We  have  lately  seen  a  whole  community  suffering 
from  the  visible  decline  of  the  thrift  of  its  fruit  trees,  which  occurred 
despite  what  was  considered  abundant  irrigation;  i.e.,  allowing  the 
water  to  run  for  a  given  length  of  time,  deemed  to  be  sufficient.  Yet 
on  being  called  in  to  investigate  the  causes  of  the  trouble,  the  station 
staff  found  that  the  irrigation  water  had  failed  to  penetrate  during  the 
allotted  time  to  any  beneficial  extent,  so  that  the  trees  were,  in  the 
main,  suffering  from  lack  of  moisture — a  fact  that  could  have  been 
verified  by  any  one  of  the  owners  concerned,  by  simply  boring  or 
digging  a  hole  or  two.  But  no  one  had  thought  of  doing  so,  and  all 
kinds  of  mysterious  causes  were  conjectured  to  be  at  work  in  the 
suffering  orchards.  A  definite  knowledge  of  the  rapidity  with  which 
irrigation  water  penetrates  downward  and  sideways  in  his  soil  should 
form  a  part  of  the  mental  equipment  of  every  irrigator,  particularly  in 
arranging  his  head  ditches.     For  in  sandy  lands  it  may  easily  happen 


11 

that  when  these  are  too  far  apart,  the  water  near  the  head  ditch  is 
already  wasting  into  the  country  drainage  at  the  depth  of  ten  or  twelve 
feet,  before  any  has  reached  the  end  of  the  furrows,  or  has  wetted  the 
lower  half  adequately.  Many  such  cases  come  under  our  observation, 
and  such  ignorance  of  the  conditions  governing  one  of  the  most 
important  factors  of  success  is  hardly  excusable  in  any  one.  Nor  is 
the  quality  of  the  water  used  indifferent  in  this  connection;  for  waters 
containing  alkali  will  fail  to  penetrate  the  soil  as  quickly  as  would 
ordinary  stream  waters. 

Preventing  evaporation. — But  supposing  the  moisture  to  have 
reached  the  depths  of  the  soil,  whether  from  rains  or  from  irrigation, 
it  is  essential  that  proper  means  be  employed  for  retaining  it  in  the 
land,  and  especially  to  prevent  evaporation.  That  this  is  best  accom- 
plished by  a  mulch  on  the  surface,  and  that  the  best  mulch  for  the 
purpose,  which  need  not  be  hauled  on  or  off  and  is  always  ready,  is  a 
surface  layer  of  loose,  well- tilled  soil,  is  now  pretty  well  understood 
by  all.  But  the  extent  to  which  the  presence  or  absence  of  such  a  non- 
evaporating  layer  influences  plant  growth  and  fruit  production  in  a 
critical  time,  is  not  so  fully  appreciated.  Plates  3  and  4,  at  end  of 
Bulletin,  give  an  illustrative  example  of  trees  and  fruit  grown  this 
season  on  adjacent  fields,  with  only  a  lane  between,  the  soil  and  all 
natural  conditions  being  absolutely  identical;  the  only  difference  being 
the  presence  and  absence  of  cultivation.  In  the  present  case  the  cultiva- 
tion was  omitted  on  principle  by  one  owner,  who  considered  cultivation 
superfluous  on  the  loose,  generous  soil  of  Alameda  creek;  while  his 
neighbor,  across  the  way,  held  the  opposite  belief,  and  had  this  season 
cultivated  to  an  extra  depth  to  conserve  moisture.  The  cultural  results 
are  sufficiently  shown  in  the  plates  and  need  no  comment,  although  it 
may  be  of  interest  to  mention  that  the  year's  growth  on  the  one  hand 
was  over  three  feet,  on  the  other  barely  three  inches.  The  effect  on 
the  fruit  is  shown  in  piate  4.  The  determination  of  the  moisture  held 
by  the  soil  in  July  to  the  depth  of  six  feet  gave  the  following  results : 


Depth  in  Soil. 

Cultivated. 

Uncultivated. 

Percent,  j   ^ 

Percent.]    Tj-P- 

First  Foot 

Second  Foot  ...  . 

6.4              128 
5.8              116 

4.3  86 

4.4  8S 

Third  Foot 

6.4              128              3.9                78 

Fourth  Foot 

Fifth  Foot 

Sixth  Foot 

6.5              130              5.1               102 
6.7               134              3.4                 68 
6.0              120              4.5                90 

Total  for  six  feet... 

6.3              756              4.2              512 

12 

The  difference  of  244  tons  per  acre  of  ground  shown  by  the  analyses 
is  quite  sufficient,  according  to  the  data  given  at  the  beginning  of  this 
bulletin,  to  account  for  the  observed  difference  in  the  cultural  result. 
The  cause  of  this  difference  was  that  in  the  uncultivated  field  there  was 
a  compacted  surface  layer  several  inches  in  thickness,  which  forcibly 
abstracted  the  moisture  from  the  substrata  and  evaporated  it  from  its 
surface;  while  the  loose  surface  soil  on  the  cultivated  ground  was 
unable  to  take  any  moisture  from  the  denser  subsoil.  This  is 
well  illustrated  by  the  familiar  fact  that  while  a  dry .  brick  will  suck 
a  wet  sponge  dry,  a  dry  sponge  (corresponding  to  the  loose  surface 
soil)  is  unable  to  take  any  water  from  a  wet  brick.  Besides,  the 
tilled  surface  soil  forms  a  non-conducting  layer  protecting  the  subsoil 
from  the  sun's  heat  and  the  dryness  of  the  air. 

In  the  East,  where  this  principle  is  well  understood,  it  is  con- 
sidered that  a  surface  layer  three  inches  in  thickness  is  sufficient  to 
afford  effective  protection.  But  what  is  adequate  in  the  region  of 
summer  rains  is  quite  insufficient  in  California  and  in  the  arid  region 
generally.  It  takes  fully  twice  the  thickness  mentioned,  and  preferably 
more,  to  afford  protection  against  the  drought  and  heat  lasting  five  or 
six  months  at  a  stretch.  Here  again  we  find  an  important  point  in 
which  our  practice  must  differ  from  that  of  the  East  and  of  the  Old 
World. 

The  beneficial  effects  of  summer  fallow  in  California  are  assuredly 
due  quite  as  much  to  the  conservation  of  moisture  brought  about  by 
the  tilled  surface  layer,  as  by  the  weathering  of  the  soil  to  which  the 
efficacy  of  the  fallow  is  commonly  ascribed.  Witness  the  fact  that 
weeds  come  up  freely  on  summer-fallow  as  late  as  August,  when 
unplowed  land  is  as  bare  as  a  barn  floor. 

Similarly  on  our  mostly  new  and  unexhausted  lands,  the  bad 
effects  of  weed  growth  are  doubtless  due  fully  as  much  to  the  waste  of 
moisture  going  on  through  their  leaves  as  to  the  competition  with  the 
crop  in  plant  food.  Hence  all  good  orchardists  are  very  careful  about 
keeping  their  ground  clean  in  summer;  but  it  must  not  be  forgotten 
that  by  doing  so  they  quickly  deplete  their  lands  of  vegetable  matter, 
which  requires  systematic  replacement  if  production  is  to  continue 
normally.  Yet  of  the  two  evils,  the  loss  of  moisture  is  more  to  be 
dreaded,  and  very  generally  in  practice  the  more  difficult  to  remedy. 


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